1. Field of the Invention
The invention disclosed relates to surge protection devices.
2. Discussion of the Related Art
Transient voltage surges above normal voltage levels may occur in power lines due to power transmission system problems, regional substation distribution problems, lightning strikes, and the like. Transient voltage surges may also occur in telephone networks and TV cable systems due to lightning strikes. Electrical and electronic equipment connected to power lines, telephone lines, and TV cable lines may sustain significant damage from high currents resulting from transient voltage surges.
A variety of surge protector devices have been developed to protect electrical and electronic equipment from the effects of transient voltage surges by either blocking or by shorting to ground high currents resulting from unwanted voltages above a safe threshold. Metal oxide varistor (MOV) devices have been used as surge protection devices, such as by connecting the device as a shunt between the hot wire or neutral wire of equipment to be protected and the ground wire. A MOV device comprises a layer of zinc oxide grains that is sandwiched between two conductive plates. The layer is electrically equivalent to back-to-back diode pairs. When a low voltage is applied across the plates, only a small current flows, however when a large transient voltage is applied, the diode junction breaks down and a large current flows. The result is that high currents resulting from transient voltage surges on power lines, phone lines, or TV cables, may be shunted to ground by the MOV device, thereby protecting the connected electrical and electronic equipment.
A problem with conventional MOV devices is that they typically are coated with an organic polymer, such as epoxy. There is an upper limit to the current that may be shunted by a MOV device before the device material overheats and destroys the device. Epoxy and other organic polymers in the coating are subject to thermal decomposition at the elevated temperatures of an overheated MOV device. The chemical decomposition of the organic polymers in the coating generates hot gases that may explosively scatter melted and charred solids and debris onto adjacent electrical components.
Some surge protection devices have addressed the problem of overheating in the MOV device they contain, by connecting the MOV device in series with a thermal cutoff device. In one known example, the surge protection device is contained in a plastic housing, and the thermal cutoff device includes a conductive arm that is spring loaded to rotate away from the MOV device. A low melting point solder is used to temporarily bond a contact at the end of the rotary arm to the MOV device. If the temperature of the MOV device rises due to a current surge, the solder melts and the arm is propelled by the spring to rotate away from the MOV device, interrupting the current surge. The components of this thermal cutoff device are relatively complex. The surge protector has the problem of its plastic housing being unable to contain explosive gases and combustion products that may be produced by the MOV device, if it is exposed to a very large transient voltage surge. Additionally, the combustion products thus produced by the MOV device tend to foul the thermal cutoff device, preventing it from interrupting the surge current.